The present invention relates to electrical fault detection; and more particularly, it relates to the detection of high impedance faults.
High impedance faults are characterized by a high impedance at the point of fault. Accordingly, a high impedance fault typically produces a small fault current level. High impedance faults can, therefore, be generally defined as those faults which do not draw sufficient fault current to be recognized and cleared by conventional overcurrent devices.
High impedance faults may result from a number of circumstances. For example, occurrences giving rise to a high impedance fault include: a tree brushing against a conductor, a broken conductor falling to the ground, contact between a conductor and a pole crossarm, and dirt accumulation on an insulator. In such occurrences, the fault current path is not clearly established, and a "bolted" fault may be delayed or may not occur at all. However, arcing commonly exists.
Clearly, high impedance faults present serious hazards.
High impedance faults are found to typically occur on electric power lines at distribution level voltages. Most utility customers in North America receive service from four-wire solidly grounded distribution feeders. The nominal phase to phase voltage on these circuits ranges from 4160 v to 13,800 v, with many operating at 12,470 v. Other distribution circuits operate at higher voltages, notably 24,900 and 34,500 v; these feeders, however, are less subject to the high impedance fault problem due mainly to their higher voltage level.
Distribution line fault clearing is commonly performed by an overcurrent sensing device such as an overcurrent relay/circuit breaker combination, a recloser or a fuse. While these devices must interrupt fault currents, they must also carry normal and emergency load currents as well as transient overcurrents caused by inrush or load pickup surges. These operating requirements suggest a tradeoff in choosing the level of current at which a device will operate. Overcurrent devices are usually set so that the lowest pickup value for operation is 150-200% of the maximum load seen by that device. This setting assures that in the case of high current faults, the feeder will be protected from burn-down while eliminating most unnecessary service interruption.
Ground relaying is used on many electric power distribution systems. Some practioners in the field believe that ground fault relaying may help indicate a high impedance fault in such systems. Since high impedance faults commonly involve a current path to ground, one indication of a fault would be an increased earth return current. In practice this proposed solution is unworkable since the sensitivity of commonly used ground relays cannot be set to detect most high impedance faults without increased risk of false indication of tripping. Furthermore, for certain utility companies who use multiple grounded systems, where earth return takes many different paths, not necessarily the return path being monitored for relaying, monitoring of earth return current to detect a high impedance fault does not work well. Also, the policy of many utility companies is to allow a certain unbalance in three phase loads (in some cases 50-75% unbalance). This produces a large "normal" neutral or earth return current and means that ground relays must be desensitized so that little advantage is obtained in detecting low-grade faults.
The relay engineer is, therefore, faced with the dilemma of setting trip levels on phase and ground relays low enough to clear some high impedance faults yet high enough to stay in service for load unbalance or inrush currents. It is usually a matter of company policy whether to use high settings and allow high impedance faults to occur, or to employ low settings and accept a higher degree of "nuisance" trips.
In many cases a fault will draw enough current to cause an overcurrent device to operate and the fault will be cleared. But if the impedance at the point of the fault is high, current may increase only a few percent above load current. The fault persists indefinitely because it is not recognized as a fault.
As a result of the inadequacies of conventional overcurrent protection schemes in clearing high impedance faults, there is substantial need for new apparatus and methods for clearing such faults.